This invention relates to a semiconductor radiation detector for use in engineering, nuclear medicine and other technical fields,
In the radiation detector for use at room temperature which comprises a compound semiconductor such as CdTe, HgI.sub.2, etc., the charge collection time or the rise time of the output signal of the preamplifier connected to the detector fluctuates greatly with the location at which the radiation has been absorbed in the detector because generally hole drift mobility .mu.h is far smaller than electron mobility .mu.e. This tendency becomes marked when measurement is conducted with gamma rays higher than 60 keV irradiating the detector through either one of the positive and negative electrodes thereof since the thickness of the detector (or the distance between the electrodes) or that of the depletion layer must be greater than 0.5 mm in view of the efficiency of detection and there is a limit to the voltage that can be impressed.
If the output signal of the preamplifier is shaped and amplified by a main amplifier and then analyzed by a multichannel pulse height analyzer (to be referred to as MCA hereinafter), the pulse height distribution becomes asymmetric with respect to the full energy absorption peak as shown in FIG. 7(a) with a tail extending at the low energy side due to the following two factors, so that it is difficult to attain a good energy resolution.
Factor (a):
Incomplete charge collection caused by the small value of the product of .mu.h.multidot..tau.h (wherein .tau.h is the mean free time for holes), that is, the tendency that positive holes are easily trapped into the hole trapping centers.
Factor (b):
The dependence of the pulse height of the output from the pulse shaping circuit on the rise time of the input signal to the circuit.
Factor (a) concerns the inherent property of the crystal of the semiconductor element and the value of .tau.h varies greatly with the quality of the crystal, or the concentration of the positive hole trapping center. For further detail reference should be made to E. Sakai: "Present State of HgI.sub.2 radiation detector" Applied Physics, 46(10) 1034(1977).
Factor (b) concerns solely the response characteristic of the electronic circuit and depends upon the type of the pulse shaping circuit (or filter) used and the shaping time constant thereof.
In FIG. 5 there is schematically shown a room-temperature radiation detector comprising a radiation detecting element 1 of a compound semiconductor such as CdTe or HgI.sub.2 and a negative electrode 2 and a positive electrode 3 on the opposite sides of the element. Usually with high resistance type CdTe or HgI.sub.2, the element 1 is the crystal itself with the electrodes 2 and 3 formed by aquadag (a trademark) painting or by vacuum-evaporating thereon a film of metal (Pd or Ge on HgI.sub.2).
With low resistance type CdTe, the element 1 is a surface barrier type or a PN junction type. In the former case the whole crystal constitutes a sensitive layer, and in the latter case the sensitive layer is the depletion layer.
A negative bias voltage is impressed on the negative electrode 2, and a preamplifier 6 is connected to the positive electrode 3 and produces a voltage signal Vp. A main amplifier (or pulse shaping amplifier) 4 receives the signal to produce an energy signal Ve, which is analyzed by an MCA.
For convenience of explanation, the detector 1 is shown sandwiched between a pair of parallel plate electrodes, with the output signal being taken out from the positive electrode 3 through direct current coupling. Any other suitable method of taking out the output signal may also be employed.
Let the thickness of the sensitive layer be 1 and the distance between the negative electrode 2 and the position at which the incident radiation has been absorbed be x. At radiation events within the area where x/l is sufficiently small, the positive holes move a short distance and the output signal is mainly caused by electrons so that the charge collecton time is short and complete charge collection is effected.
At radiation events with a greater value of x/l, however, the positive holes move a greater distance so that the charge collection time becomes longer and the previously mentioned factor (a) or the incompleteness of charge collection becomes marked. This tendency can be expressed by a solid line curve Vpmax in FIG. 6 as the dependence of the pulse height Vpmax of the output voltage from the preamplifier 6 upon x/l. In FIG. 6, instead of the information about the position of absorption, the charge collection time or the rise time tr of the output signal of the preamplifier 6 may be taken on the abscissa.
The shaping amplifier 4 has the previously mentioned factor (b), that is, the pulse height Vemax of its output signal depends upon the rise time of the input signal thereto. Generally, with the pulse height of the input signal being kept constant, the pulse height Vemax of the output signal tends to decrease as the rise time of the input signal increases. Therefore, Vemax/Vpmax decreases as the rise time increases. The dependence of Vemax on tr can be shown as a dot-and-dash line curve in FIG. 6.
When the energy signal Ve such as mentioned above caused by irradiation with single energy gamma rays is analyzed by a pulse height analyzer, the pulse height distribution curve obtained is asymmetric as shown in FIG. 7(a) with a tail traling at the low energy side of the full energy absorption peak. As shown in FIGS. 7(b) to 7(d), the pulse height distribution varies with the position at which the radiation is absorbed, and the energy absorption peak is shifted toward the low energy side as the value of x/l increases.
The shaping amplifier 4 has four functions, that is, (1) amplification, (2) reduction of noise in the circuit, (3) production of pulses with a small width for improvement of the count rate characteristic, and (4) production of pulses with such a shape that can be processed with ease. The shape that makes it easy to process the pulse means that the pulse has a comparatively flat peak with respect to the time axis when pulse height analysis is to be conducted, for example.
There are various types of pulse shaping circuits or filters which can be used in the amplifier 4. Except the function of amplification, these filters have advantages and disadvantages with respect to the other functions. Circuit noise originates in the semiconductor detector and the input stage of the preamplifier and can be classified into series noise, parallel noise, flicker noise and others. The shaping time constant of the shaping circuit (which will be referred to merely as the time constant hereinafter) is defined more or less differently in different types of filters. In all of them, generally the smaller the time constant is, the greater the series noise becomes and the greater the time constant is, the greater the parallel noise becomes. The total circuit noise is minimized with such a time constant that both series and parallel noises become of the same level. Flicker noise does not depend upon the time constant.
On the other hand, the greater the time constant is, the longer the dead time becomes and the lower the count rate becomes. Therefore, the time constant is usually determined at a compromise between reduction of circuit noise and the count rate characteristic.
If the fluctuation of the charge collection time is so great that the previously mentioned factor (b), that is, the dependence of the pulse height of the output from the pulse shaping circuit upon the rise time tr of the output from the preamplifier 6 is not negligible, it is necessary to set the time constant (for example the peaking time) to a sufficiently long time as compared with the charge collection time, so that it is not always possible to select such a time constant as to satisfy the condition for minimum noise and that the count rate is reduced.
Usually a quasi-gaussian filter is often used as the pulse shaping circuit. This type of filter generally comprises a combination of a single differentiator and a plurality of integrators and enables substantial reduction of circuit noise with a relatively simple circuitry. In FIG. 9 the pulse shaping amplifier is shown as an amplifier 30 comprising a differentiator/amplifier 31 and an integrator/amplifier 32. Usually, the differentiator/amplifier 31 employs a single CR differentiator with pole-zero cancellation. In the integrator/amplifier 32 an RC integrator or a low-pass active filter is usually used as the integrator. The filter may comprise a single integrator or a plurality of integrators. The integrator/amplifier 32 often includes a baseline restorer. The amplifier 30 often includes a pileup rejection circuit, etc. With the quasi-gaussian filter used in the shaping circuit, however, disadvantageously the previously mentioned factor (b) is relatively great. The dependence of Vemax/Vpmax on tr is shown in FIG. 8 by way of example as dot-and-dash line curves a, a' and a", with the time constant (peaking time) increasing in the order of a, a' and a".
The single delay line clipping filter has an advantage that the previously mentioned factor (b) or dependence is very small as compared with other filters having substantially the same time constant. The clipping filter, however, has a disadvantage that it inherently has great circuit noise.
It has been reported that in a system such as a large volume germanium detector of the coaxial type whose charge collection time fluctuates (and in which the previously-mentioned factor (a) or incompleteness of charge collection is rarely recognized), by using a pseudo-trapezoidal filter which is a kind of time-variant filter or by shaping the signal by a quasi-gaussian filter and then integrating the shaped signal by a gated integrator for a predetermined period of time it is possible to eliminate the previously mentioned factor (b) or dependence with a short time constant thereby to achieve a good energy resolution and high count rate characteristic. For further detail reference should be made to the following documents:
(2) V. Radeka; "Trapezoidal Filtering of Signal from Large Germanium Detectors at High Rates." IEEE Trans. Nucl. Sci., NS-19(1) 412(1972). PA0 (3) F. S. Goulding, et al.; "Signal Processing for Semiconductor Detectors." IEEE Trans. Nucl. Sci., NS-29(3) 1125(1982). PA0 (4) L. T. Jones; "The Use of Cadmium Telluride .tau. Spectrometers in Monitoring Activity Deposited in Nuclear Power Stations" Rev. Phys. Apl., 12,379(1977). PA0 (5) R. Kurz; "A Novel Pulse Processing System for HgI.sub.2 Detectors." Nucl. Instr. and Meth., 150, 91(1978). PA0 (6) M. Finger, et al.; "Energy Resolution Enhancement of Mercuric Iodide Detectors." IEEE Trans. Nucl. Sci., NS-31(1)348(1984). PA0 (7) D. Ortendahl, et al.; "Operating Characteristics of Small Position-Sensitive Mercuric Iodide Detectors." IEEE Trans, Nucl. Sci., NS-29(1)784(1982). PA0 (8) C. Ortale, et al.; "Mercuric Iodide Detectors." Nucl. Instr. and Meth., 213,95(1983).
For comparison, the dependence of Vemax/Vpmax on tr with respect to the pseudo-trapezoidal filter is shown in FIG. 8 as a solid line curve b. This curve is obtained by integrating the output of the quasi-gaussian filter which produces the curve a for a predetermined period of time. Vemax/Vpmax is kept almost constant for about the same period of time as the integration time. The dead time, however, becomes longer than in the case of the curve a and about the same as in the case of the curve a'.
With respect to such compound semiconductor type radiation detectors for use at room temperature as mentioned above, there have been proposed two methods (A) and (B) of improving by means of circuitry the degradation of the energy resolution caused by the previously mentioned factors (a) and (b).
Method (A):
According to this method only those radiation events the x/l of which is sufficiently small are measured as shown in FIG. 7(b).
Method (B):
Since the full energy absorption peak is shifted to the low energy side as x/l increases as shown in FIGS. 7(b), 7(c) and 7(d), the signal caused by a radiation event with a greater value of x/l is amplified by a greater amplification degree (or the output Vemax is corrected by adding thereto or subtracting therefrom an amount which is a function of both x/l and Vemax), thereby to make the energy absorption peak less dependent on x/l.
Method (A) is described in, for example,
According to this method, at each radiation event the rise time tr of the output from the preamplifier is measured as the depth information x/l of the position of the radiation absorbed in the detector, so that those signals with a long rise time are omitted. This method (A) remarkably improves the energy resolution but greatly deteriorates the detection efficiency.
On the other hand, method (B) advantageously improves the energy resolution without appreciably lowering the detection efficiency as reported in the following:
In the method disclosed in literature (5) which is similar to the method disclosed in literature (4), the rise time tr of the output from the preamplifier is measured at each radiation event as the information about the depth x/l of the position of the radiation absorbed. The circuit employs a single delay line clipping filter, which causes large circuit noise and complication of the circuitry with two constant fraction discriminators and a time-to-amplitude converter to be provided.
In the method disclosed in literature (6), the output from the preamplifier is applied to two kinds of pulse shaping amplifiers, one of which is a quasi-gaussian filter with a great time constant and the other is a fast pulse processing circuit, so that by using the output signal S (slow) from the former filter and the output signal S (fast) from the latter circuit the information x/l about the depth of each radiation event in the detector is obtained to correct the signal S (slow).
The fast pulse processing circuit is so arranged that integration is conducted for about 100 ns upon passage of 50 ns from the rising point of the output signal of the preamplifier in order to obtain only signals caused mostly by movement of electrons. Therefore, the ratio S (fast)/S (slow) tends to decrease as x/l increases. Disadvantageously, however, the fast pulse processing circuit is complicated in structure and large errors are likely to be involved in measurement of gamma rays at relatively low energies such as for example 60 keV-140 keV.
In the methods described in documents (5) and (6) the time constant of each filter is set to a sufficiently great value to substantially eliminate the influence of the previously mentioned factor (b).
In the above explanation, detrapping of positive holes from the hole trapping center is neglected. Practically the situation is a little more complicated due to the influence by such detrapping. In principle, however, the situation can be improved by methods (A) and (B) although in method (B) the degree of correction and the nonlinearity of correction with respect to x/l are more or less different from in the above case where detrapping is negligible.
One object of the invention is to provide a semiconductor radiation detector which uses a compound semiconductor element as a radiation detecting element for use at room temperature and which is capable of improving the energy resolution which would otherwise be deteriorated by the incompletness of the charge collection (the previously mentioned factor (a)) and the dependence of the output of the pulse shaping circuit on the rise time of the input thereto (the previously mentioned factor (b)) caused by the use of a compound semiconductor element, without substantially deteriorating the detection efficiency.
Among various expected applications of such semiconductor radiation detectors as mentioned above is a two-dimensional position-sensitive radiation detector for use particularly with gamma rays having energies above 60 keV for diagnosis in nuclear medicine. Several position-sensitive radiation detectors which employ HgI.sub.2, etc. as a detecting element have been proposed in, for example,
In designing such radiation detectors the following problems must be solved:
(I) Low energy resolution particularly at energies above 60 keV,
(II) low uniformity of sensitivity,
(III) low count rate,
(IV) difficulty in making the detector head (particularly when a higher resolution and a wide field of view are required),
(V) high manufacturing cost of the detector head, and
(VI) relatively short life of the detector head (caused by degradation of the detecting element).
The problem (I) originates in the previously mentioned factor (a), that is, the incompleteness of charge collection and factor (b), that is, the dependence of the output from the pulse shaping circuit on the rise time tr of the input thereof.
The factor (a) primarily concerns the inherent quality of the crystal of the semiconductor detecting element. The lower the quality of the crystal is, the lower the energy resolution becomes, with resulting degradation of the uniformity of sensitivity (problem (II)).
To prevent this, a crystal of very high quality must be used for the detecting element in a large amount to provide a great number of detecting elements, with resulting increase in the manufacturing cost (problem (V)).
Deterioration of the characteristic of the detecting element caused by polarization effect occurring in connection with the factor (a), destruction by radiation and other causes (that is, reduction of the energy resolution and shifting of a peak in the energy spectrum) results in shortening of the useful life of the detecting element (problem (VI)).
To improve the reduction of the energy resolution caused by the previously mentioned factor (b), generally the time constant of the pulse shaping circuit must be set to a great value with resulting degradation of the count rate characteristic (problem (III)).
Another object of the invention is, therefore, to provide a two-dimensional position-sensitive radiation detector which comprises a plurality of radiation detecting elements of a compound semiconductor and which has an improved energy resolution, uniformity of sensitivity and count rate characteristic, and which can be used without trouble even with more or less variation in quality of the semiconductor crystal of different detecting elements, with resulting reduction of the manufacturing cost, and which has a long life of the detecting elements even with more or less deterioration of the crystal of the detecting elements by reducing its influence over the detector.
The invention will be described in detail with reference to the acompanying drawing.